Ethylene propylene copolymers made with metallocene catalysts are known. Many such copolymers are intermolecularly heterogeneous in terms of tacticity, composition (weight percent comonomers) or both. Further, such polymers may also, or in the alternative, be compositionally heterogeneous within a polymer chain. Such characteristics may be, but are not always, the result of multiple reactor schemes or sequential addition of polymer.
The elasticity, flexural modulus and tensile strength of such copolymers, when considered in the aggregate, may not reach a satisfactory level for use in commercial elastomeric operation.
U.S. Pat. No. 5,747,621 suggests fractionable reactor blend polypropylenes, directly obtainable from the polymerization reaction of propylene having 30 to 90% by weight of a boiling n-heptane fraction, soluble in xylene at 135° C. In Table 2 of this document, the only fractionation disclosed, each of the solvents appears to be at its boiling point. Further, reference to this table shows that the diethyl-ether fraction has no melting point (amorphous).
In the journal articles Science, Vol. 267, pp 217–219 (1995); Macromolecules, Vol. 31, pp 6908–6916 (1998); and Macromolecules, Vol. 32, pp 8283–8290, pp 3334–3340 and pp 8100–8106, propylene polymers with similar characteristics as those disclosed in the above discussed U.S. Pat. No. 5,747,621 are made and fractionated. The polymers are made with bis(aryl indenyl) or bisindenyl metallocene catalysts. In these journal articles, these polymers are fractionated in boiling ether and heptane, leaving a portion of the polymer insoluble in either. The polypropylenes are stated to be compositionally heterogeneous in terms of tacticity and molecular weight.
U.S. Pat. No. 5,504,172 suggests a propylene elastomer that has properties such that:                (a) the elastomer contains propylene units in an amount of 50 to 95% by mol and ethylene units in an amount of 5 to 50 % by mol;        (b) a triad tacticity of three propylene units-chains consisting of head-to-tail bonds, as measured by 13C NMR, is not less than 90.0%; and        (c) a proportion of inversely inserted propylene units based on the 2,1-insertion of a propylene monomer in all propylene insertions, as measured by 13C NMR, is not less than 0.5%, and a proportion of inversely inserted propylene units based on the 1,3-insertion of a propylene monomer, as measured by 13C NMR, is not more than 0.05%.        
U.S. Pat. No. 5,391,629 suggests block and tapered copolymers of ethylene with an α-olefin. The copolymers are made by a process of sequentially contacting ethylene with an α-olefin monomer in the presence of an activated cyclopentadienyl catalyst system.
EP 0 374 695 suggests ethylene-propylene copolymers and a process for preparing them. The copolymers have a reactivity ratio product, r1r2, between 0.5 and 1.5 and an isotactic index greater than 0 percent. The copolymers are produced in the presence of a homogeneous chiral catalyst and an alumoxane co-catalyst.
There is a commercial need therefore for an ethylene propylene copolymer that will show a melting point and an excellent balance of elasticity, flexural modulus and tensile strength. It would further be desirable if such polymers could be produced at higher polymerization temperatures.
It is known that temperature affects the polymerization involving the stereo regular polymerization of alpha-olefins, in particular propylene. Under similar polymerization conditions the increase in the polymerization temperatures leads to both a drop in molecular weight as well as a loss in the tacticity of the alpha olefin residues along the chain. This effect exists for both the homopolymerization of the 1-olefins as well as copolymerization of 1-olefins with ethylene, or other alpha-olefins. These changes in the characteristic of the polymer are detrimental to certain end uses of the polyolefin. However, there are commercial incentives in raising the polymerization temperature since this improves the throughput of the polymerization reactor. This would necessarily lead to better economics for production for these polymers if physical attributes of the polymer product, such as tacticity and molecular weight, could meet or exceed the properties now achieved at lower temperatures.